Chemical Tests for Biological Molecules (2.1.1)

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What is a qualitative test?
A simple lab test that will show whether or not a particular substance is present. The results are descriptive.
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What is the test for starch?
The iodine test.
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How does the iodine test work?
Sample of substance placed in well of spotting tray. Add a few drops of iodine solution.
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What is the colour change in the iodine test?
If starch is present, the colour change will be from yellow/brown to blue/black.
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What is the test for reducing sugars (monosaccharides and most disaccharides)?
The Benedict's test.
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How does the Benedict's test work?
Place 3cm cubed of test solution into test tube. Add equal volume of Benedict's solution. Heat in water bath at about 80 degrees C for 3 minutes.
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What is the colour change for the Benedict's test?
If a reducing sugar is present, the colour change will be from blue to brick red.
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How do you test for non-reducing sugars?
If 1st attempt at Benedict's test gives a negative result, boil solution with HCl for 2 minutes (this hydrolyses the disaccharide into its 2 monosaccharides). Add some hydrogen carbonate powder to neutralise acid. Redo Benedict's test.
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Why is sucrose not a reducing sugar?
When glucose and fructose join to make sucrose, the ability to donate electrons is tied up in the bond, so there is no reaction with Benedict's solution.
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What is the test for proteins?
The Biuret test.
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How does the Biuret test work?
Take sample to be tested, and add an equal volume of Biuret A solution. Trickle a few drops of Biuret B solution down the side. These chemicals react with peptide bonds present.
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What is the colour change in the Biuret test?
If protein is present, the colour change will be from pale blue to lilac.
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Why would the Biuret test give a negative result with a solution of amino acids?
The Biuret chemicals react with peptide bonds, which will be found in a polypeptide chain, but not when it's individual amino acids.
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What is the test for lipids?
The emulsion test.
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How does the emulsion test work?
Mix small quantity of sample under test with ethanol. This will dissolve any lipids present. Pour this into some water in a separate test tube. If lipid is presents, a cloudy white emulsion will be seen near the water layer.
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What is quantitative biochemical testing?
Where you can quantify how much of a particular substance is present, such as numerical results with units. This is much more accurate.
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How can the Benedict's test be made quantitative?
The Benedict's test results in a brick-red precipitate. The more reducing sugar present, the more precipitate formed, and the less blue Benedict's solution left. If the precipitate is filtered out, the blue solution remaining can be measured.
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How do you make a calibration curve to estimate concentration of reducing sugars?
Take range of known concentrations of reducing sugar, do Benedict's test on each, then filter to leave blue solution. Use colorimeter to measure amount of light passing through solutions. Plot transmission of light against conc. of reducing sugars.
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Card 2

Front

What is the test for starch?

Back

The iodine test.

Card 3

Front

How does the iodine test work?

Back

Preview of the front of card 3

Card 4

Front

What is the colour change in the iodine test?

Back

Preview of the front of card 4

Card 5

Front

What is the test for reducing sugars (monosaccharides and most disaccharides)?

Back

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